The present invention relates to an improvement in weft measuring devices used in shuttleless looms, and in particular to feeding-measuring devices feeding weft yarns to a storage chamber used for temporarily storing a predetermined length of weft yarns before the yarn is cast into the shed.
Shuttleless looms, and in particular those looms in which the weft is cast through the shed by pressurized means (air jet, or water jet), or by the energy communicated to the mass of the yarn itself, are all equipped with a measuring device which permits the length of the picks to be determined as exactly as possible, particularly in order to reduce wastes due to the yarn coming out on the sides of the formd fabric. Such measuring devices should also be usable for yarns liable to have different characteristics, which is particularly the case in multi-pick looms where yarn measuring is intermittent.
The measuring devices proposed up to now can be divided into two major classes (1): the drum measuring devices where the length is determined by a number of spires, and (2) the unwinding measuring devices where the yarn is squeezed between two rotating rollers. The first devices, which, mechanically, are more complicated, are quite accurate, whereas the second type, although simpler in design, lack accuracy and are difficult to use on multi-pick looms where different types of yarns are cast at a very specific rate.
This disadvantage is explained by the structure of these measuring devices and by the way they work. Indeed, the length of the yarn in these measuring devices is determined either by an intermittent clearance between the rollers, or by the temporary introduction of the yarn between the rollers which are then held in permanent contact. The accuracy of the measurement by successive clearances between the rollers is independent of the thickness of the yarn, which may be variable, and of its coefficient of friction on the rollers, which may vary in relation to the orientation of the fibers. Moreover, and especially when working at high speeds, the movable roller may bounce on the driving roller, giving an erroneous measurement of the length of the yarns.
In the measuring devices where the rollers are kept in permanent contact, inaccuracy is due to a number of damaging factors present mainly when the yarn penetrates between the rollers. To help this penetration, it has been proposed to provide a slight peripheral chamfer as described in U.S. Pat. No. 3,833,028. But the effect, in this case, is similar to that of the two surfaces coming closer together in the case of a system of rollers with temporary clearance between the rollers, and as such it has the same disadvantages as those indicated above, especially when yarns of different natures and possibly of different thickness, are alternately worked. Another problem which arises with this type of measuring device is the disturbing effect produced by the air stream created by the rotation of the rollers when the yarn is introduced between them. Indeed, the surface of the casings of the rollers carries a fine layer of air or limit layer at its peripheral speed, which air cannot therefore go through the gripping line and escapes laterally, thus tending to push the yarn back when it is introduced between the rollers.
Finally, the adjustment of this type of measuring device in relation to the width of the fabric to be produced (modification of weaving width) is difficult.
It has also been proposed (French Pat. No. 1 492 449), in another field than weaving, namely in the spinning field, to use a yarn feeding device comprising two rotating rollers, one driving, the other pressing. In this device the pressing roller is split on one of its edges in order to transport the yarn from the repair plane (knotting) to the plane where the yarn is squeezed between the rollers.
Such a solution, however, is not transposable to the measuring devices used on looms as it does not permit synchronization with the weaving cycle.